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The Mass Distribution of 238U Photofission Fragments B ISSN 00271349, Moscow University Physics Bulletin, 2013, Vol. 68, No. 4, pp. 279–287. © Allerton Press, Inc., 2013. Original Russian Text © B.C. Ishkhanov, A.A. Kuznetsov, 2013, published in Vestnik Moskovskogo Universiteta. Fizika, 2013, No. 4, pp. 15–22. The Mass Distribution of 238U Photofission Fragments B. C. Ishkhanova and A. A. Kuznetsovb a Faculty of Physics, Moscow State University, Moscow, 119991 Russia b Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia email: [email protected] Received April 1, 2013; in final form, April 18, 2013 Abstract—The mass distribution of the 238U photofission fragments formed under γquanta action is ana lyzed in the range of excitation energies of a fissile nucleus from 5 to 20 MeV. The influence on the ratio of asymmetrical and symmetrical photofission of the 238U structure and excitation energy of a fissile nucleus is discussed. A combined analysis and the comparison of the behavior of the asymmetrical and symmetrical modes of fission under the action of γquanta was conducted for the first time. The results we obtained are compared with the prediction of the multimode model of the dependence of separate fission modes on the excitation energy of a fissile nucleus. Keywords: photofission, mass distribution of fission fragments, gammaactivation analysis. DOI: 10.3103/S0027134913040073 INTRODUCTION several works. Mainly, the works on the study of photo fission mass distributions were performed on beams of Beginning from the first works on nuclei fission, the γ mass distribution was interpreted as the superposition braking quanta; hence, difficulty arises in the com parison of results obtained under the conditions of of two fission modes, viz., symmetrical and asymmet γ rical [1]. Symmetrical fission at great excitation ener various geometries and for different converters of gies is mainly explained by the liquiddrop model. The quanta. In this work we studied the behavior of fission modes under the action of braking γquanta in the concept of the peak/plateau ratio that reflects the 238 manifestation of shell effects during fission was intro range of excitation energies of the U nucleus from 5 duced in fission physics. Asymmetrical fission of to 20MeV. The main factors that determine the char actinide nuclei at low energies is explained by the acteristics of fission products are the properties of a fis shellnucleus structure when one fragment has the sile system and the excitation energy of a compound number of protons Z and neutrons N close to the nucleus. magic numbers Z = 50 and N = 82. The development of the shell model for deformed METHOD AND PROCESSING nuclei [2] and the method of calculation of shell cor OF EXPERIMENTAL RESULTS rections to the nucleus energy [3] explained the asym metry of mass distributions as passage of the nucleus The mass distributions of 238U photofission at elec during fission through a doublehumped barrier. The tron energies of the accelerator at 19.5, 29.1, 48.3, and calculations of the potential energy surface of a fissile 67.7 MeV were obtained in [6]. The experiment was nucleus in deformation multidimensional space dem carried out on the beam of braking γquants of an onstrated that a nucleus on the path from the first sad RTM70 microtron (Research Institute of Nuclear dle point to separation into two fragments can pass Physics of Moscow State University) [7]. The braking through several trajectories, i.e., potentialenergy spectrum of γquanta was formed on a 2.5 mm thick minima [4, 5]. For most actinide nuclei there are three tungsten target. A target produced from a natural 235U dominating fission modes: a symmetrical superlong and 238U isotope mixture was located immediately SL mode and asymmetrical STI and STII modes. before the tungsten target. The content of 238U These asymmetrical modes are connected with neu (99.27%) in the natural isotope mixture was higher tron shells of N = 82 fragments for SNI and N = 88 for than 235U; hence, all the results related to 238U photo STII. fission. The spectrum of braking phonons was calcu Despite the considerable investigation of the lated using the GEANT4 program [8]. Spectra of photofission process, the dependence of photofission γquanta of residual activity of irradiated uranium tar modes on the nuclear excitation energy has not been get were measured on a germanium γspectrometer. studied. The contributions of various components of The method of the experiment was described in previ the photofission mass distribution were estimated in ous papers [6, 9]. γActivation experiments allow one 279 280 ISHKHANOV, KUZNETSOV 3/2(+) 97 36Kr 3/2(+)169.9 ms 97 37Rb β− n 25.1% 1/2+ 426 ms n 97 38Sr β− 0.005% 0% 8 > < 20% (27/2)IT 3523.3 142 ms β− % .7 0 (9/2)+ <667.51 1.17 s IT β− 1/2 0 3.75 s 97 39Y β− n 0.055% (3.75 s) < 0.08% (1.17 s) 1/2+ 16.91 h 97 40Zr β− IT 1/2 743.35 52.7 ms + 9/2 0 72.1 ms 97 41Nb β− 5/2+ 97 42Mo Fig. 1. The net of decay of nuclei–isobars with mass number A = 97. to obtain the independent and accumulated yields of N λ Y = 10 1 , separate photofission products after momentary neu 1 –λ t ()1 – e 1 1 trons are emitted. Forty chains of fission of nuclei– (1) isobars in the range of mass numbers A = 80 – 160 S N = , were analyzed. The fission chain of nuclei–isobars A = 10 –λ ()t – t –λ ()t – t ()1 2 1 1 3 1 97 is shown in Fig. 1. k1 e – e The accumulated yield is the summed number of where N10 is the number of radioactive nuclei at the definite isotope nuclei that form immediately as a moment of radiation completion, S is the area of the result of both the division and decomposition of pater photo peak in the spectrum of residual activity during nal nuclei. In the case of the formation of division measurement, t1 is the irradiation time, t2 is the time at products with mass number A = 97 (Fig. 1) one can which measurement starts, t is the time that measure determine the accumulated Y yield of zirconium iso 3 1 ment is completed, λ is the decay constant, k is a 97 1 1 tope 40Zr formation (Fig. 2) coefficient equal to the product of the detector effi MOSCOW UNIVERSITY PHYSICS BULLETIN Vol. 68 No. 4 2013 THE MASS DISTRIBUTION OF 238U PHOTOFISSION FRAGMENTS 281 − β − decay + fission Fission Fission Y1 Y2 N , λ N , λ 97 − 10 1 97 − 20 2 97 40Zr(β , 16.91h) 41Nb(β , 72.1 min) 42Mo(stable) 97 97 97 Fig. 2. Formation of nuclei–isobars of 40Zr , 41Nb and 42Mo upon decay. ciency and the quantum yield of γquanta at γtransi determine all the yields in the chain of the β–fissions tions. of isobars with the given mass number A. However, the If the yield of the parent nuclei–isobars is known, yields of separate photofission products can be esti one can determine the independent yield of the mated using the charge distribution, namely the yield daughter nucleus i.e., the number of the definite iso dependences of separate photofission products on tope nuclei that form immediately as a result of fission. mass number. The independent Y yield of 97Nb nucleus formation The charge distribution is well approximated by the 2 41 Gauss function [10]: (Fig. 2) only as the result of photofission is determined 97 () (), MY A []()2 from the accumulated yield of 40Zr formation accord IY A Z = exp – ZZ– P /C dZ, (3) ing to the formula πC λ λ – 1t1 – 2t1 where IY(A, Z) is the independent yield of a photofis λ N λ ()λ1 e ()1 e 2 20 2 – – 1 – sion product with given A and Z, MY(A) is total yield of Y2 = λ – Y1λ , – 2t1 – 2t1 isotopes with a given mass number, Z is the most 1 – e ()λ2 – λ1 ()1 – e p probable charge in the charge distribution, and C is the S N10λ1 width of the charge distribution. N20 = λ ()λ () + (2) – 2 t2 – t1 – 2 t3 – t1 λ λ k ()e – e 2 – 1 The value of most probable charge Zp for every 2 nucleus–isobar chain was calculated using the ratio λ () λ () – 1 t2 – t1 – 1 t3 – t1 [10]: N10λ2 ()e – e – λ ()λ (), λ λ – 2 t2 – t1 – 2 t3 – t1 Z ==Z ± ΔZ , Z ()Z /A ()A + ν , , (4) 2 – 1 ()e – e p UCD p UCD F F LH where ZF and AF are the charge and mass of the fissile where λ , λ are decay constants of 97Zr and 97Nb iso 1 2 40 41 system, ZUCD is the most probable charge based on the topes, Y is the accumulated yield of the 97Zr isotope assumption that the ratio of the proton number and 1 40 neutrons in light and heavy fission fragments is the 97 Δ formation; Y2 is the independent yield of 41Nb isotope same as in a fissile nucleus [11], Zp is the charge polarization calculated based on systematism [10]. formation as the result of fission, and N10 and N20 are 97 97 The + and – signs correspond to the light and heavy the quantities of Zr and Nb nuclei at the moment ν ν 40 41 fragments, respectively. L and H are the numbers of that irradiation is completed. neutrons that are emitted by light and heavy fragments The independent formation yield as the result of and estimated according to the method [12]: the photofission of separate isomeric states, such as ν ν () L = 0.531 + 0.062 AL + 143 – AF , the isomeric 97Nbmeta state, can be determined in a (5) 41 ν = 0.53ν + 0.062()A – 143 .
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